1. Field of the Invention
The present invention relates to a film comprising an interpolymer of ethylene and 3-substituted C4-10 alkene, wherein the interpolymer is made using a catalyst system comprising a Ziegler Natta or chromium oxide catalyst. The invention also relates to a process for the preparation of the film and to laminates and articles comprising the film.
2. Description of the Related Art
Polyethylene is widely used in the manufacture of films, often for use in packaging applications.
The use of films to form laminates for use in the packaging industry is, for instance, well known. Laminates are used to form a range of articles, for example, food containers, stand up pouches and product labels. Films used for this purpose need to possess a certain combination of properties. Specifically the films need excellent tear strength in order that during production, and in use, articles made from the film, do not fail. The films should also possess impact strength and tensile strength to make the films usable. Without adequate mechanical properties such as impact strength, thicker films have to be made which is economically unattractive and in some cases less aesthetically appealing. It is also important that films have adequate optical properties such as low haze and high gloss. Stiffness is also an important property, particularly if they are to be used in the preparation of stand up pouches. Obtaining films having a desirable combination of tear strength and stiffness is often a challenge.
Another common application of polyethylene films in the packaging industry is in the formation of bags or sacks. These are used, for example, in the packaging of food stuffs such as cereals and crisps, as well as much heavier materials such as sand, cement mix, compost, stones etc. It is often desirable for the bags to be transparent in order that their content can be easily determined. More significantly, however, and especially in the case of heavy duty sacks (e.g. bags and sacks designed for the packaging of materials up to 25 kg, or even 50 kg in weight) the key requirement is that they possess good mechanical properties such as impact strength and tensile strength, and tear strength. This is necessary as bags and sacks are usually transported on pallets, one on top of the other. Hence the total load on at least some of the sacks is extremely high, e.g. in the region of 1000 kg or more, in some instances. Moreover such sacks are rarely handled with care and must be able to withstand conventional transportation conditions without tearing. Additionally a certain level of stiffness, e.g. for stability on pallets, is usually desirable.
Films having attractive combinations of properties, especially tear strength and mechanical strength, particularly impact strength and tensile strength, as well as reasonable optical performance are therefore highly desired for use in the packaging industry. The difficulty often encountered, however, is that those polymer properties that minimize, e.g. haze, are often those that are detrimental to, e.g. impact strength. Additionally those polymers possessing low haze and reasonable impact strength, often have poor stiffness whilst those polymers possessing high tear strength often have poor stiffness and in addition tend to have high melt viscosities making melt processing difficult.
It is therefore common to utilize interpolymers and/or blends of polymers in the manufacture of films to try to provide the desired balance of film properties. Thus ethylene may be copolymerized with a comonomer such as 1-butene in order to obtain a polymer yielding films having increased dart drop strength and tear resistance. In other words comonomers are generally used to tailor the properties of a polymer to suit its target film application. There are vast numbers of commercially available films that are made from ethylene and 1-butene copolymers that provide advantages over ethylene homopolymer films.
A film manufactured from ethylene/1-butene, for example, typically has improved impact strength (e.g. dart drop) compared to an ethylene homopolymer of the same density as dart drop strength generally increases with the increasing molecular weight of the comonomer. Its optical properties are also usually excellent. On the other hand, however, films made from ethylene/1-butene copolymers often do not have as high a tear strength or as strong mechanical properties as desired. Often this is disadvantage is compensated for by making films thicker than is desirable. There is therefore a trade off between polymer properties such as impact strength, tensile strength, tear strength and melt viscosity and the thickness of the film that is usable.